In order to ensure security in telecommunications networks, user equipments (UEs) connected to an IP Multimedia Subsystem (IMS) network have a secure association with the IMS. In current standards, this secure association is between the UE and the Proxy Call Session Control Function, P-CSCF, of the IMS network. The secure association is created when the terminal registers with the IMS.
The P-CSCF is the contact point between the IMS and the UE, i.e. all communications between the UE and the IMS are routed via the P-CSCF to which the UE is registered. Therefore, any malfunction of the P-CSCF will cause a loss of service for UEs registered to the P-CSCF. One possible failure mode of the P-CSCF is the loss of the secure association with the UE. In the case of originating sessions (i.e. session requests sent by the UE), the UE will recognise that a malfunction has occurred and perform re-registration. Re-registration may result in the UE being assigned to a new P-CSCF, following a P-CSCF discovery step. In the case of terminating sessions (i.e. session requests sent to the UE), the session request or ongoing session will be lost.
There are several current solutions which address the problem of terminating sessions. These solutions operate by means of packet core network (i.e. the network between the P-CSCF and the UE) mechanisms to force the UE to re-register. For example, notifications may be generated within the IMS and passed down to the packet core network, or the packet core network itself may detect that the P-CSCF has failed. These procedures require the restoration to be performed outside of the IMS, which requires nodes in the packet core network to be adapted to handle the procedures, and may involve further latency.
In order to ensure low failover times if a P-CSCF malfunctions, redundant nodes may be provided which can “step in” for a failed P-CSCF. However, in order to ensure that the redundant node is able to maintain the secure association, IPSec data must be backed up frequently from the primary node.
Recently, cloud-based P-CSCF architectures have been developed, where the duties of a single P-CSCF are shared between multiple physical or virtual instances. For load balancing or other reasons, it may be desired to relocate UEs to other instances of the P-CSCF, which could cause a change of P-CSCF IP address. Since the IPSec protocol contains security elements which are specific to the IP address of the sender and receiver, this would cause the secure association to be lost. However, since there would not be an obvious failure of the P-CSCF, it would not trigger existing restoration procedures in most cases (e.g. unless the UE attempted to initiate an outgoing session). It is therefore desirable to have a smoother means of handling such transitions so that the IMS service can be made more independent from the underlying IP structure.